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An invisible shield that makes objects vanish has been proposed by scientists who say their idea obeys the laws of physics.

The shield would be a metallic structure, much like the shielding used by the Romulans in Star Trek who hid their spaceships by pushing a button.

Electronic engineers Andrea Alù and Professor Nader Engheta from the University of Pennsylvania say their proposed "plasmonic shield" is just an idea at this stage.

They report their calculations on the arXiv physics website, which is owned and operated by Cornell University in New York.

They say their shield would use the special properties of plasmons, clouds of electrons that move within metallic material.

Plasmons would be used to cancel out the light that is scattered from, or bounces off, objects, which would normally allow an object to be seen.

Alù and Engheta reason that if the frequency of the shield's so-called plasmon resonance, or the frequency at which the electron cloud oscillates, is close to the frequency of the light, then the object disappears.

The mystery of the vanishing chocolates

"If you tune the wave of the light to the same frequency of the plasmon it will absorb the light," says Dr Ian Falconer, an Australian plasma physicist from the University of Sydney.

"The light effectively goes straight through it and whatever is in the shield can't be seen."

Falconer says the concept is like chocolates.

"If you had an M&M in which the outer coating was plasmon you wouldn't see the M&M because the light would effectively go through it, but you would see the plate it was on," he says.

"The light actually is absorbed but the plasmon coating makes it come out the back as if there was nothing there."

The shield would need fine-tuning

However, critics say the shield would have to be finely tuned for each object it hides and a shield would only work for a particular wavelength of light.

And the effect only works when the wavelength of the light being scattered is roughly the same size as the object.

Falconer says the technology couldn't be used to shield humans because the wavelength of natural light is far too short compared with the size of a human.

It could theoretically be used to hide spaceships or other large objects from sensors using long wavelength radiation instead of ordinary light, the researchers say.

More realistically, it could be used to create anti-glare materials or invisible probes for use in microscopy, say the researchers, whose research received funding from the US Defense Advanced Research Projects Agency.